In general, optical disks, especially optical recording disks have a structure in which a recording layer is formed on a substrate. Recording and reproducing operation is carried out by directing light, typically a laser beam to the recording layer through the substrate. Since the substrate has to be transparent, it is usually formed of glass or a resin. Most often, resinous substrates are used in the prior art optical disks for light weight and ease of formation of tracking grooves and recording pits.
To achieve a high transfer rate, it is recently required to rotate the disks at high revolutions during recording and reproducing operation. However, resinous substrates are less stiff or rigid so that dynamic axial runout occurs during operation, and more focus tracking errors occur during high revolution driving at 1,800 r.p.m. or higher, especially 3,000 r.p.m. or higher. To meet the recent demand for high revolution driving, the use of substrates of glass having high mechanical strength must be considered. Unfortunately, glass substrates are prone to break when disks are accidentally dropped or struck. In particular, substrates of chemically tempered glass having high strength can break into scattering small fragments.
To protect the recording layer from mechanical failures as by scratching and marring, a protective plate must be provided on the recording layer bearing side of the substrate. Alternatively, a pair of substrates are joined such that the recording layer is interposed therebetween. In order to prevent separation between a glass substrate and a protective plate due to differential thermal expansion, a choice of glass will be effective for the protective plate. If a protective plate of glass is bonded to the glass substrate, however, the resulting disk would have a total weight in excess of a critical level. It is then difficult to establish a necessary number of revolutions, especially a steady high number of revolutions. Further, the problems associated with glass substrate failure are left unsolved with the use of such a protective plate.
It would be possible to bond a resinous substrate or protective plate to a resinous substrate for preventing any mechanical damage to the recording layer and for preventing separation due to differential thermal expansion. This approach, however, cannot overcome the problem of focus tracking errors during high revolution driving.
These problems, especially the problems associated with high revolution driving can be solved by forming one of a pair of substrates or one of the substrate and the protective plate from glass and the other one from resin. There was known or available in the prior art no optical disk in which one of the paired substrates or one of the substrate and the protective plate is formed from glass and the other one is formed from resin. It was believed that a bond between glass and resin having different coefficients of thermal expansion was not recommended for optical disks which could be exposed to severe storage conditions.
U.S. Pat. No. 4,503,531 discloses that resinous substrates are joined with a hot melt adhesive having a softening point of up to 140.degree. C., a dry tensile bond strength of at least 1 kg/cm.sup.2 at 20.degree. C., and a melt viscosity of up to 1,000 poise at 160.degree. C. Also, Japanese Patent Application Kokai No. 165050/1989 discloses a hot melt adhesive for bonding a pair of substrates. However, these adhesives are effective only in forming a bond between resinous substrates or between a resinous substrate and a resinous plate having approximately equal coefficients of thermal expansion, but unsuitable for bonding a glass substrate and a resinous substrate or protective plate having different coefficients of thermal expansion. More particularly, if an optical disk having a glass substrate bonded to a resinous substrate or protective plate with an adhesive is stored under severe conditions at high or low temperatures, for example, at temperatures varying between -20.degree. C. and 55.degree. C., then stresses and strains are induced in the substrate and protective plate and even in the recording layer, protective coating, and adhesive layer due to differential thermal expansion. Therefore, in the prior art optical disks using conventional adhesives for bonding purposes, separation can occur between the substrates, between the substrate and the protective plate, between the substrate and a protective coating or another layer, or between such layers. Most conventional adhesives fail to provide reliable optical disks by integrally bonding a glass substrate and a resinous substrate or protective plate.
A similar situation occurs even when a pair of substrates are formed of the same type of material. In particular, single side recording type optical recording disks in which a resinous substrate having formed thereon necessary layers including a recording layer is integrally bonded to a resinous protective substrate have a higher coefficient of thermal expansion than glass-based disks and asymmetric deflection, stresses or strains can occur in the substrates. Under severe thermal conditions, for example, at temperatures varying between -30.degree. C. and 65.degree. C. at a rate of 20.degree. C./sec., the conventional adhesives cannot prevent separation from occurring between the substrates, between the substrate and the protective substrate, between the substrate and the layer, or between the layers.